As a light source to be used for a processing application or a laser display, a high-power laser light source is attracting attention. In an infrared area thereof, a solid laser such as a YAG laser, and a fiber laser using a fiber to which a rare earth such as Yb or Nd is applied are developed. On the other hand, in red and blue areas, a semiconductor laser using GaAs, GaN, or the like is developed, and an increase in output power thereof is also considered. However, as for a green area, it is still difficult to generate green color directly from a semiconductor, and it is general to obtain green color by wavelength-converting an infrared light emitted from the above-mentioned solid laser or fiber laser by using a non-linear optical crystal.
Particularly when lithium niobate (LiNbO3:LN) crystal is used as the non-linear optical crystal, a high conversion efficiency can be obtained because of a large non-linear optical constant of the crystal, and the construction of the device is simplified. Therefore, a quasi phase matching (QPM) wavelength conversion element that is formed by applying a polarization inversion technique to the lithium niobate crystal is often used for a wavelength conversion optical device having an output power of about hundred mW.
In a wavelength conversion optical device having an output power of several W class, a non-linear optical crystal such as lithium triborate (LiB3O5:LBO) crystal or potassium titanyl phosphate (KTiOPO4:KTP) crystal is used. The LBO crystal has such characteristics that crystal destruction or deterioration due to a fundamental wave or a generated second harmonic wave hardly occurs. However, since the non-linear optical constant of the LBO crystal is small, it is necessary to constitute a resonator and place the crystal in the resonator to achieve high conversion efficiency of the wavelength conversion optical device, and thus the device constitution is complicated and minute control is required. On the other hand, since the KTP crystal has a non-linear optical constant larger than that of the LBO crystal, high conversion efficiency of the device can be achieved without constituting a resonator. The KTP crystal, however, has such drawback that crystal destruction or deterioration due to a fundamental wave or a generated second harmonic wave easily occurs.
By the way, since the quasi phase matching element using the LN crystal (QPM-LN element) has a larger non-linear optical constant than that in the case of using the KTP crystal, it enables high-efficiency and high-power wavelength conversion. However, since the QPM-LN element is required to concentrate light energy to a narrow region, substantially, crystal destruction or deterioration due to a fundamental wave or a generated second harmonic occurs more likely in the LN crystal than in the KTP crystal. That is, when a harmonic wave of several watts (W) is obtained by the QPM-LN element, the large non-linear optical constant of the LN crystal causes that an ultraviolet light (third harmonic wave) which is a sum frequency of an infrared light as a fundamental wave and a wavelength-converted green light (second harmonic wave) occurs even when the phase matching condition is not satisfied, and the generated ultraviolet light (third harmonic wave) induces absorption of the generated green light (second harmonic wave), leading to saturation of the green light output, and crystal destruction.
In order to solve this problem, it is proposed that a non-linear optical crystal for waveform conversion is constituted in two stages, and a fundamental wave that is not converted by the first-stage wavelength conversion is again used in the second-stage wavelength conversion, as described in Patent Document 1 and Patent Document 2. In this case, total conversion efficiency can be increased by the first-stage and second-stage wavelength conversions, and further, interaction between the fundamental wave component and the second harmonic wave component can be reduced to suppress generation of the third harmonic wave that is ultraviolet light.
Patent Document 1: Japanese Published Patent Application No. Hei.11-271823
Patent Document 2: Japanese Published Patent Application No. 2005-10739